Add initial README

Author: felipecrs

README.md

@@ -1 +1,40 @@
-# docker-on-docker-shim
+# docker-on-docker-shim
+
+This is a shim that remaps volume mounts when running docker on docker. Aimed to fix <https://stackoverflow.com/questions/31381322> with no hassle.
+
+## Compare it yourself
+
+Using the [`fixdockergid`](https://github.com/felipecrs/fixdockergid) image to test, where the `/usr/local/bin/fixdockergid` is only available within the container:
+
+```bash
+# This does not work
+docker run --rm -u "$(id -u):$(id -g)" -v /var/run/docker.sock:/var/run/docker.sock felipecrs/fixdockergid docker run --rm -v /usr/local/bin/fixdockergid:/fixdockergid ubuntu test -f /fixdockergid
+
+# A non-zero exit code indicates that it did not work
+echo $?
+
+# This does
+docker run --rm -u "$(id -u):$(id -g)" -v /var/run/docker.sock:/var/run/docker.sock ghcr.io/felipecrs/dond-shim docker run --rm -v /usr/local/bin/fixdockergid:/fixdockergid ubuntu test -f /fixdockergid
+
+# A zero exit code indicates that it worked
+echo $?
+```
+
+## Installation
+
+Considering your container already has the `docker` CLI installed at `/usr/bin/docker`:
+
+```dockerfile
+FROM my-image-with-docker-cli
+
+USER root
+
+ARG DOCKER_PATH="/usr/bin/docker"
+ARG DOND_SHIM_REVISION="main"
+RUN mv -f "${DOCKER_PATH}" "${DOCKER_PATH}.orig" && \
+  curl -fsSL "https://github.com/felipecrs/docker-on-docker-shim/raw/${DOND_SHIM_REVISION}/docker" \
+    --output "${DOCKER_PATH}" && \
+  chmod +x "${DOCKER_PATH}"
+
+USER non-root-user
+```